Electromagnetic interference shield for electronic devices

ABSTRACT

An EMI shield for personal computers, cellular telephones, and other electronic devices is constructed from thermoformable polymeric material which is then metallized on all surfaces by vacuum metallization techniques to provide an inexpensive, lightweight, yet effective EMI shield.

[0001] This application is a division of co-pending application Ser. No.08/463,297 which was filed Jun. 5, 1995, which was acontinuation-in-part of Ser. No. 08/254,250 filed Jun. 6, 1994.

BACKGROUND OF THE INVENTION

[0002] This invention pertains to shielding apparatus for containinghigh frequency electromagnetic radiation within a personal computer,cellular telephone, or other electronic instrument.

[0003] Electromagnetic compatibility (EMC) is a broad term used todescribe electromagnetic interference (EMI), radio frequencyinterference (RFI) and electrostatic discharge (ESD), and the aboveterms are often used interchangeably.

[0004] The fact that electronic devices are both sources and receptorsof EMI creates a two-fold problem. Since electromagnetic radiationpenetrating the device may cause electronic failure, manufacturers needto protect the operational integrity of their products. Secondly,manufacturers must comply with the regulations aimed at reducingelectromagnetic radiation emitted into the atmosphere. Proper design isnecessary to prevent the device's function from being disrupted byemissions from external sources and to minimize its system's emissions.

[0005] Today, plastics are replacing metals as the material forelectronic enclosures since plastics offer increased design flexibilityand productivity with decreased cost. The switch from metal to plasticsas a housing material for electronic equipment has contributed toconcern over EMI shielding. Plastics are insulators, so EMI waves passfreely through unshielded plastic without substantial impedance orresistance. Additionally, ever increasing device miniaturization and theincrease in clock speeds of microprocessors used in computing devicesmakes it more difficult to handle the EMI pollution these fastercomputers generate. So a variety of technologies using metal/polymercombinations and composites are being used as a shielding material inelectronic devices.

[0006] Current methods for shielding of electromagnetic interference(EMI) include the use of metal housings, metal filled polymer housings,metal liners for housings, and conductive coatings for the interior ofrigid polymer or composite housings.

[0007] Metal coatings for rigid plastic housings are applied through useof conductive paints or through application of metal platings applied bychemical plating (electroless plating), by electroplating, or throughvacuum metallization. In addition, metal foils with adhesive backingsmay be applied to the inside of plastic cases for electronic instrumentsto achieve shielding requirements. Zinc Arc spray techniques are alsoavailable to apply a metal coating to a plastic housing.

[0008] Another shielding material is provided through the use of metalfibers sintered onto a polymeric substrate as is taught in U.S. Pat. No.5,226,210, and commercially produced as #M 610D ThermoformableEMI-shielding material by the Minnesota Mining and Manufacturing Companyof St. Paul, Minn.

[0009] Each of the current shielding methods have shortcomings. Themajor disadvantages of plating are its high cost, complex processcycles, and its application is limited to only certain polymer resins.Metal-filled resins for injection molding suffer from poor conductivitycompared to metals. The conductive polymer resin is very expensive andcomplex shape molding is difficult from flow and uniformityperspectives.

[0010] Three general types of conductive metal-bearing paints are ingeneral use. Silver paints have the bast electrical properties but, theyare extremely expensive. Nickel paints are used for relatively lowattenuation applications and are limited by high resistance and poorstability. Passivated copper paints have moderate cost and lowerresistivity, but also lack stability. All paint applications havedifficulties with coating uniformly, blow back in tight areas andrecesses depending on part complexity, and application problems whichcan lead to flaking. Paints also fail ESD testing over 10 KVA.

[0011] EMI shielding through the use of metal cases for the personalcomputer or other electronic device may not always be desired because ofconcerns about weight and aesthetics, with weight being a seriousconcern for laptop computers or portable and handheld devices of anytypes. The use of a metal shroud to line a plastic case improves overthe metal case in aesthetic and design concerns for the outside of thehousing but results in an increased assembly step and little weightminimization. Metal also lacks the ability to be formed into complexshapes often taking up unnecessary room adjacent to the circuitry andassembled electrical components.

[0012] The use of coated plastic housings for electronic devices,including microcomputer and cellular telephones, may not provide asuitable solution when one considers that personal computers currentlyoffered may operate at clock speeds of 100 MHz which gives rise toopportunities for EMI generation not previously confronted in thepersonal computer industry. Further, the ever increasing clock speeds ofthe personal computers being offered makes effective shielding more andmore challenging since any breach in the shield which has one dimensionin excess of 0.23 inch may allow substantial EMI leakage, causing theunit to fail United States Federal Communication Commission standards.

[0013] The use of metallic coatings on plastic housings presents certainmanufacturing and service concerns. A slipped tool used during assemblyor a repair can cause a scratch in the metal coating of sufficient sizeto cause a slot antenna, thereby making the case totally useless, andthereby leading to a costly item being discarded with little feasibilityfor successful recycling.

[0014] The seams of a metal plated plastic housing will act like slotantennae unless the housing sections are conductively joined by the useof overlapping joints, conductive gaskets, or conductive tape. When thehousing must be opened for a repair or retrofit, it can be understoodthat some of the conductive interconnection may be degraded by theactivity of disassembly.

[0015] Further background on prior art methods and characteristics ofshielding methods may be examined in “EMI/RFI Shielding Guide” publishedby the GE Plastics Division of the General Electric Company, and in “theDesigner's Guide to Electromagnetic Compatibility” by Gerke & Kimmel,Supplement to EDN Magazine, Volume 39, No. 2, (Jan. 20, 1994) to both ofwhich the reader is directed.

SUMMARY OF THE INVENTION

[0016] This invention pertains to EMI shielding for personal computers,cellular telephones and other electronic devices which are subject toPart 15 of the FCC Rules. A thermoformable polymeric sheet is formedinto an enclosure sized and shaped to enclose an EMI emitting subsystemor component. The thermoformed polymeric enclosure is then metallized onall or selected surfaces by vacuum metallizing techniques where thethermoformed enclosure is placed in a vacuum chamber, treated with anionized gas, and then metallized by the use of aluminum or other metalbeing vaporized by use, for example, of a current-passing tungstenfilament, or other vaporization techniques. The enclosure is rotatedwithin the chamber to allow metallization of all desired surfaces.Masking may be employed when certain regions or surfaces are preferrednot to be metallized. The enclosure is thereby provided with wallshaving a polymeric substrate provided on desired surfaces with a vacuummetallized layer. The vacuum metallized layers are of sufficientthickness to make the surfaces of the enclosure electrically conductive.The enclosure is formed in the shape and size necessary to house andshield the EMI emitter; for example in the case of a personal computer,the enclosure may serve as a thin-walled case within the rigid outerhousing of the computer. Alternatively, the enclosure may be formed tofit as an insert within a device's housing as a substitute for a metalinsert shield, or the enclosure may be shaped and sized to contain onlycertain components which are emitters of, or susceptible to, EMI. Gangsof metallized enclosures may be devised with electrical isolation asdesired provided by gaps in the metallization layers. Differentelectronic devices will require varying degrees of attenuation orshielding effectiveness. The enclosure may be coated on all surfaces orselectively coated for certain applications.

[0017] Thermoformed shapes have previously been vacuum metallized withthin-film coatings (350 to 1000 angstroms or 0.035 to 0.10 microns) butonly for their reflective metallic appearance. Conventional thin-filmvacuum metallizing is not adequate to dissipate EMI. Existing equipmentfor metallizing thermoformed shapes for ornamental reflective appearancepurposes is not suitable for application of relatively thick thin filmas is required to provide suitable surface impedance to allow effectiveEMI dissipation.

[0018] Many polymeric materials are thermoformable. Formability,thickness, melt strength, shrinkage, flame retardency, and otherproperties are factors determined by the end user of the finishedproduct. Extruded roll or sheet materials suitable for thermo forminginclude Acrylonitrile-Butenate-Styrene (ABS), polystyrenes, cellulosepolymers, vinyl chloride polymers, polyamides, polycarbonates,polysulfones, and olefin polymers such as polyethylene, polypropylene,polyethylene terephthalate glycol (PTG) and methylmethacrylate-acrylonitrile (co-polymers).

[0019] Use of these polymers with additional fillers such as carbonblack, graphite, and metal fibers, add to the shielding effectivenessfor absorbing more of the lower electromagnetic wave lengths.

[0020] The polymeric enclosures are not metallically coated until afterthe thermoforming process. Because the forming process stretches ordraws the material into corners and recesses, it would also draw or thinthe metallic coating making its uniformity vary in different areas onthe formed shape if coatings were applied prior to forming.

[0021] After forming, metallic coatings may be applied to the shapes bya variety of vacuum deposition techniques such as thermal evaporation,cathode sputtering, ion plating, electron beam, cathodic-arc, or vacuumthermal spray.

[0022] Because a thermoformed enclosure is used, the shield is ofreduced weight and if damage occurs to the thermoformed shield duringmanufacturing or repair of the electronic device, a less costlyreplacement item is needed.

[0023] The use of interlockable enclosure bodies which may snap togetheror otherwise be mechanically held in assembled state, permits the wallsof the shield to be in conductive contact and reduces or eliminates theneed for conductive tape or conductive gaskets while providing aneffective EMI shield. Further securing means may be employed, such as byuse of conductive adhesive, laser welding, or heat sealing.

[0024] It is accordingly an object of the invention to provide an EMIshield which may be thermoformed into a desired shape with metallizationapplied on all surfaces of the shield.

[0025] It is another object of the invention to provide an EMI shieldwhich provides an easy-to-manufacture shield with excellent attenuationof the strength of electric or magnetic fields.

[0026] Another object of the invention is to provide an inexpensive EMIshield that will not be totally degraded by a scratch on one surface ofthe shield.

[0027] Another object of the invention is to provide an EMI shield whichis light weight.

[0028] Another object of the invention is to provide an EMI shield whichmay be nested for shipment.

[0029] Another object of the invention is to provide an EMI shield withsuperior conductive wall coupling structure.

[0030] Another object of the invention is to provide an EMI shield whichwill not need application of conductive tape or gaskets to provideadequate shielding.

[0031] Another object of the invention is to provide an EMI shield whichincreases resistance of the shielded components to corrosive atmosphericconditions.

[0032] These and other objects of the invention will become understoodfrom a review of the detailed description of the invention whichfollows.

DESCRIPTION OF THE DRAWING FIGURES

[0033]FIG. 1 is an exploded perspective view of a laptop personalcomputer having the shield invention installed therein.

[0034]FIG. 2 is an enlarged view of a typical cross section of asidewall of the preferred embodiment shield invention.

[0035]FIG. 3 is a schematic view of the typical apparatus used forapplying metal deposition to the polymer thermoforms of the preferredembodiment.

[0036]FIG. 4 is a plan view of the preferred embodiment of the inventionin its unfolded arrangement.

[0037]FIG. 5 is an enlarged perspective view of the interconnectingedges of the preferred embodiment shield invention.

[0038]FIG. 6 is an enlarged cross section of the engagement between acover member and a base member of an alternate embodiment of the shieldinvention.

[0039]FIG. 7 is an enlarged cross section of a second alternateembodiment of the shield invention showing the cover thereof in phantomin an open position.

DETAILED DESCRIPTION OF THE INVENTION

[0040] Referring to the drawing figures and in particular to FIG. 1, theinvention 2 is shown in place as a component of a laptop personalcomputer 4. Bottom case 6 of computer 4 is provided with power supplymodule 7 stationed therein. Invention 2 encloses the mother board of thecomputer 2, including the central processing unit, memory storage chips,input-output circuit components and the like (not shown). Top case 8overlies invention 2 when invention 2 is placed within bottom case 6 ofcomputer 4. Top case 8 includes keyboard 10 and visual display 12 whichare interconnected to associated circuitry housed in invention 2 byleads 14. Power supply 7 and input/output ports 9 are electricallyconnected to associated circuitry housed in invention 2 by cables 16.

[0041]FIG. 2 discloses a cross section of a wall of invention 2, showinga polymeric substrate 25 having conductive metallization layers 27 and29 applied thereto by vacuum metal deposition techniques. Each of layers27 and 29 are a relatively thick, thin film of metal, preferably ofaluminum, copper, or silver. In the preferred embodiment, aluminum isused, and is applied to the polymeric substrate 25 after the polymericsubstrate has been thermoformed into a desired enclosure shape and thenits surface is modified by bombardment by an ionized gas in an evacuatedchamber or by other means suitable to increase surface tension of thesubstrate 25. The substrate 25 is then placed in an evacuated chamberwhere a metal is vaporized and deposited on the substrate 25 on thesurfaces thereof, through rotation of the substrate 25 about itself andabout the metal vapor source. Substrate 25 has been earlier formed intoa desired shield shape before application of the metallization layers 27and 29 in order to achieve a uniform thickness of metallization over thesurfaces of substrate 25. By thermoforming substrate 25 beforesubjecting it to the metallization step, problems with thinning of themetallization layers 27 and 29 at corners, bends and the like, whichmight occur if the substrate were formed after metallization is applied,are avoided. If desired, certain regions of substrate 25 may be maskedto prevent deposit of metal film on those regions.

[0042] By applying a relative thick film (between 1.0 and 50 micronsthick), which has a surface impedance of less than one ohm per squareper inch, a suitably conductive layer of metallization is achieved whichprovides a low surface impedance and hence effective EMI attenuation.The application of metallization layers 27 and 29 to opposing sides ofsubstrate 25 increases the EMI attenuation achieved.

[0043]FIG. 4 discloses the preferred embodiment of the invention 2 inits unfolded state. Polymeric sheet material is thermoformed into adesired shield blank 30 by conventional methods. Blank 30 comprisesfirst section 18 and second section 20 interconnected by hinge region22, all of which are formed from continuous polymeric sheet of generallyuniform thickness. By use of thermoformable material, it can beunderstood that light weight is realized and that unassembled shieldblanks 30 may be nested for shipment.

[0044] From FIGS. 4 and 5, it can be seen that the edges 24 of firstsection 18 of blank 30 are formed to fit in complementary engagementwith the edges 26 of second section 20 of blank 30. In particular, edges24 of first section 18 are provided with shoulder recesses 19 whereinridges 21 of second section 20 are receivable, such that the periphery34 of second section 20 is overlapped by the periphery 32 of firstsection 18 when first section 18 and second section 20 are folded abouthinge region 22 into engagement. By this overlap, EMI shielding is madesubstantially thorough as the touching engagement of overlapping secondsection 20 on first section 18 provides electrical conductivity betweenthe sections. Conductive adhesive or conductive tape may be added to theseam formed between first section 18 and second section 20 to ensuresufficient EMI shielding in the seam region. Hinge region 22 provides aconductive path between first section 18 and second section 20. To aidin reducing gaps in EMI shielding effect, conductive adhesive 36 may beapplied to flanges 38 and 40 of blank 30, which will come into abutmentwhen enclosure bodies 18 and 20 are folded about hinge 22 for edgewiseengagement.

[0045]FIG. 5 illustrates further the novel mechanical locking means ofthe preferred embodiment EMI shield 2. Boss 39 is formed upon flange 40of first section 18 and is engageable with recess 41 formed in flange 38to provide additional retention forces when first section 18 and secondsection 20 are engaged.

[0046]FIG. 3 discloses apparatus for vacuum deposition of metalliccoating on thermoformed blank 30 which is placed on carrier 62 inevacuable chamber 60. Chamber 60 is evacuated and a gas, including a gasfrom the group including Argon, Nitrogen, Oxygen, CF₆ and SF₄, is passedinto chamber 60, excited by an electric charge, and the resultingionized gas serves to modify the surfaces of blank 30. Chamber 60 isagain evacuated and carrier 62 is caused to revolve around tungstenfilament 64 which is energized electrically to provide energy tovaporize metal 66, the molecules of which travel from filament 64 andare deposited on blank 30. Blanks 30 may alternatively be retained toplanetary mount 68 which rotates about itself as it revolves aboutfilament 64 in the direction of arrows a. Control 67 is associated withchamber 60 to cause evacuation of the chamber, introduction of gas forsurface modification of the blank, and energization of the filament.

[0047]FIG. 6 illustrates another embodiment of the shield inventionwherein a thermoformed case 130 is formed of formable sheet polymer. Lid132 is likewise thermoformed of sheet polymer into a complementaryshape. Lid 132 is provided with spring element 134 about its periphery,spring element 134 being formed upon lid 132 and being urged intotouching engagement with inner surface 138 of case 130. After case 130and lid 132 are suitably thermoformed, they are passed through a vapormetal deposition operation where a metal film is deposited on selectedsurfaces, including all surfaces thereof if desired. In the embodimentof FIG. 6, metal film of thickness between 1.0 and 50.0 microns isdeposited upon outer surface 140 of lid 132 and a similar metal layer isvapor deposited upon outer surface 142 of case 130 and upon innersurface 138 of case 130. Inner floor surface 144 of case 130 ispolymeric, having been masked to prevent deposit of metallizationthereon. In this embodiment, conductivity of surface 144 has beenavoided in order to prevent interference with surface circuitry of acomponent carrying board which may be installed within the shield ofFIG. 6.

[0048]FIG. 7 discloses another alternative embodiment of the shieldinvention wherein shield body 102 comprises a thermoformed polymer base104 with a hinged cover member 106. Shield body 102 is provided withprojections 108 upon the upper area of first sidewall 110. Cover 106 isfixed by hinge 122 to case 104 and is provided with indents 112 whichare formed in the outer leg 114 of U-shaped recess 116. Recess 116extends around the periphery of cover 106 and is provided to permittouching engagement of outer leg 114 with the sidewalls of base 104. Atregion 124, for example, conductive surface contact is provided betweenbase 104 and cover 106.

[0049] Shield 102 is first thermoformed into the desired shape fromsheet polymer material and then a metallic layer is vapor deposited onall or selected surfaces of shield 102. The metallic layer is suitablythick to provide excellent surface conductivity thereby providingexcellent EMI attenuation.

[0050] It can be further understood that thermoformed shapes such asenclosure bodies 18 and 20 of FIG. 4 may be ganged together byinterconnected webs, such as hinge 22 of FIG. 4, wherein a metaldeposition layer is applied to the outer surfaces of shapes 18 and 20respectively while no metallization is applied to hinge 22, such resultbeing effected by application of masking to hinge 22 before it is passedinto the evacuated chamber where metal is to be vapor deposited thereon.The resulting gang of metallized shapes may then be used to provide EMIshielding to discrete though adjacent components or component groups ona circuit board where the components or groups are separated by adistance substantially equivalent to the dimension of the web (hinge 22)between sections 18 and 20. The absence of conductive metal coating onhinge 22 prevents conduction of electrical signals from one shape to theother while providing an efficient system of creating inexpensive EMIshield gangs.

[0051] Vacuum deposition is the vaporization and subsequent condensationof any metal or compound onto a substrate in a substantial vacuum.Although commonly referred to as a single process, the deposition ofthin films by vacuum evaporation consists of several distinguishablesteps, namely: transition of a condensed phase, which may be solid orliquid, into gaseous state; metal vapor traversing the space between theevaporation source and the substrate at reduced gas pressure;condensation of the vapor upon arrival on the substrates. Accordingly,the theory of vacuum evaporation includes the thermodynamics of phasetransitions from which the equilibrium vapor pressure of materials canbe derived, as well as the kinetic theory of gases which provides modelsof the atomistic processes. Further investigation of the sometimescomplex events occurring in the exchange of single molecules between acondensed phase and its vapor led to the theory of evaporation, aspecialized extension of the kinetic theory. The distribution ofdeposits on surfaces surrounding a vapor source can be derived. Thekinetic aspects of condensation processes are a topic in their own rightrelating to nucleation and growth phenomena. Vacuum deposition and itsapplications have benefited from various disciplines which havecontributed toward solutions of practical problems. These pertain to thedesign of suitable vapor sources and the development of specialtechniques for the evaporation of metals, alloys, compounds, and avariety of vacuum systems with specialized process controls andautomation.

[0052] The transition of solids or liquids into gaseous state may betreated as a macroscopic or as an atomistic phenomenon. The formerapproach is based on thermodynamics and yields a quantitativeunderstanding of evaporation rates, interactions between evaporants andtheir containers, stability of compounds, and compositional changesencountered during the evaporation. The atomistic approach is derivedfrom the kinetic theory of gases and provides models which describeevaporation processes in terms of properties of individual particles.The latter theory also applies to the evacuation of vessels.Thermodynamic and kinetic theories are treated in various textbooks andoften times research data will vary from text to text.

[0053] Coating properties depend on deposition procedures which becomequite complex and must be monitorized closely from cycle to cycle tomaintain consistency. Every detail is important and details ofprocedures are usually proprietary. Relevant coating properties relatedto deposition of any metal consist of (1) coating structure, (2)internal stress and (3) adhesion. As any metallic coating grows fromultra-thin (10 angstroms, 0.001 microns) to relatively thick, thin-films(up to 500,000 angstroms, 50 microns), various coating zones result fromthe interaction of the basic processes that occur during deposition,coating flux transport, and surface and bulk diffusion. When a coatingis applied to a substrate, stresses usually develop within the coatingand at the interface, consisting primarily of superimposed thermal andintrinsic stresses. The mechanisms of adhesion between metal coatingsand organic polymers (thermoformed substrates) are typical of those thatare generally observed, classed as mechanical or interlocking, weakboundary layer, chemical, and electrostatic. Film discoloration,substrate warpage, even total part destruction, can easily occur if theproduct or vaporization time and current is too high, the load is notlarge enough, or part placement is not optimal. Firing too long or at anexcessively high current can burn or thermal warp the substrates.

[0054] When vacuum deposition is used to coat thin wall thermoformedshapes, care must be taken in the deposition process.

[0055] During the metal deposition cycle heat is generated and thedistance from the deposition source to the thin-walled thermoformedpieces becomes a critical factor. In an evacuated chamber, there islittle conduction or convection of heat but the radiant energy from theevaporant source can distort, warp, and even melt the polymer forms,especially in corner, or deep draws where the film is drawn to itsthinnest dimension. Thermal properties and wall thickness of thethermoformed film, heat output of the evaporant source, distance fromsource to substrate, duration of vaporization, and rotation speed of thesubstrate are all variables which need consideration.

[0056] The critical factors which need to be considered when vacuummetallizing thermoforms are:

[0057] 1) Thermal properties and thickness of the wall of the substrate.

[0058] 2) Method and heat dissipation of evaporation.

[0059] 3) Duration or time cycle of deposition.

[0060] 4) Pressure within the chamber.

[0061] 5) Type and amount of material being vaporized.

[0062] 6) Speed or movement past the vaporization source.

[0063] 7) Substrate distance from the evaporant source.

[0064] Transfer of heat within a solid, liquid, or gaseous media fromone medium to another can occur by conduction, convection, or radiation.When coating within a vacuum, conduction and convection becomeinsignificant and radiant heat transfer is the significant attribute tocontrol. Thermoformed shapes not corresponding to any definite or simplegeometry generally do not constitute cases of unidimensional heattransfer. The method of vaporization and time also affects the amount ofenergy released from the vaporization source.

[0065] The following table shows a comparison of the typical energiesfor the different types of vacuum deposition sources. Type of DepositionElectrical Vapor State Thermal Energy Thermal Evaporation ElectricallyNeutral 0.1 to 0.3 eV Ion Beam High Negative Potential 10 to 40 eV (3000to 4000 V) Sputtering High Negative 10 to 40 eV (Variable)

[0066] The radiant heat output from both ion beam and sputtering alongwith the extended cycle times needed to deposit relatively thick thinfilms make these methods less commercially feasible than the thermalevaporation method. Thermoformed polymers have much lower thermalconductivities and thermal diffusities than injection molded parts andare more susceptible to damage from the radiant heat and cycle timesnecessary for the other two methods of evaporation, namely sputteringand ion beam.

[0067] For practicable deposition rates, source materials should beheated to a temperature so that its vapor pressure is at least 1 Pa(10⁻² torr) or higher. Table A represents these temperatures for severalcommon elements. TABLE A TABLE TEMPERATURE FOR VAPOR PRESSURE OF 1 Pa(10⁻²torr) Temperature Element C. F. Aluminum 1150 2100 Beryllium 12452270 Cadmium 265 510 Carbon 2460 4460 Chromium 1400 2550 Copper 12602290 Gold 1400 2550 Indium 945 1730 Lead 715 1320 Magnesium 440 825Manganese 940 1720 Molybdenum 2350 4260 Nickel 1530 2780 Platinum 20903790 Silicon 1470 2680 Silver 1630 2970 Tantalum 3060 5540 Tin 1250 2280Titanium 1740 3160 Tungsten 3230 5840 Zinc 345 650 Zirconium 2400 4350

[0068] Aluminum is the most common element used for vacuum depositionand because of its various properties, aluminum is used in as many asninety percent of the deposition applications. Although at a pressure of1 Pa its vapor temperature is 2100° F., it is normally vaporized at atemperature over 3000° F. (1649° C.). Actual results have provenaluminum vaporized most efficiently at vapor temperatures between 3300°F. to 3600° F. This is approximately 50 percent above the actual vaportemperatures (at 1 Pa). Other elements react relatively similarlyalthough they may have their own idiosyncratic characteristics. Whenvapor temperatures are very high and vapor source temperatures are veryhigh, as hot gaseous vapor flux travels from the source towards thesubstrate, its temperature rapidly dissipates with the distancetraveled. Surface measurements made on thermoforms twelve inches fromthe source indicate that the vapor flux temperature is reduced toapproximately 185° F. to 190° F. (85° to 90° C.) as it condenses on thepart surfaces. It is interesting to note that vapor source temperaturesare normally ten to twenty-five times greater than the heat deflectiontemperature of the substrate polymers being processed.

[0069] The relative ease of vacuum coating any part is related to itsshape or configuration, its position relative to the vapor source (vaporflux) and its distance from the source. Graph A shows that maximumcoating thickness is obtained at a centerline to the substrate. As theangle of incidence increases, the thickness decreases rapidly droppingto less than fifty percent at a forty-five degree angle. At more than aforty-five degree angle, the coating density and adhesion are also verypoor.

[0070] Distance from the vapor source to the part to be coated is themost important process variable and the easiest to control. Although thevapor source may be movable in some equipment designs, it is usuallyfixed. Substrate tooling and fixturing set-ups are normally convenientlyarranged to be adjustable to accommodate distance changes for differingsubstrates. It is well known in the art that when decorative coatings(less than 0.1 microns thick) are applied to injection molded parts(parts with wall thickness greater than 0.040 inches) a distance fromeight inches to twelve inches between vapor source and part should bemaintained to prevent warpage and burning. The thicker the part andhigher the thermal properties, the closer the part may be placed to thevapor source.

[0071] Our experiments with thin-walled thermoforms (0.006 to 0.020inches thick) and EMI functional coatings (over 1.0 microns thick) havedeveloped the results listed in the following table B. A minimumdistance of twelve inches should be maintained for even the highestthermal property films. TABLE B DISTANCE FROM VAPOR SOURCE TO SUBSTRATEIN INCHES FOR VARIOUS THERMOFORMED RESINS¹ HEAT DEFLECTION MINIMUM RESINTEMP. ° F. WALL DISTANCE FROM SYMBOL NAME (UNFILLED RESIN)² THICKNESSVAPOR SOURCE PI Polyimide 460-480 .006 in. 12 to 14 inches LCP LiquidCrystal Polymer 428-465 .006 in. 12 to 14 inches PEI Polyetherimide387-392 .010 in. 12 to 14 inches PS Polysulfone 345 .010 in. 12 to 14inches PC Polycarbonate 250-270 .010 in. 14 to 16 inches PBTPolybutylene Terphthalate 248-266 .010 in. 14 to 16 inches PPSPolyphenylene Sulfide 221 .012 in. 16 to 18 inches ABSAcrylonitrile-Butadiene-styrene 170-220 .012 in. 16 to 18 inches HIPSHigh-Impact Polystyrene 170-205 .012 in. 16 to 18 inches PETGGlycol-Modified Polyester 145-151 .015 in. 18 to 20 inches PVC PolyvinylChloride 130-150 .015 in. 18 to 20 inches PP Polypropylene 120-140 .015in. 18 to 20 inches

[0072]²ASTM D 648, Heat deflection temperature under flexural load, 264P.S.I.

[0073] Polyimide (PI) and liquid crystal polymer (LCP) represent themost heat resistant thermoforms coated. A six millimeter wall thicknessthermoformed shape of PI placed at a distance of twelve inches from thevaporization source was found to be the limiting combination of wallthickness and distance to evaporant source which could be achievedwithout heat distortion. It should also be noted that specialized orcustom thermoforming equipment is necessary to form shapes from thesematerials because they require higher forming temperatures with longerheating and cooling cycles.

[0074] The more commonly used thermoformed materials such as high-impactpolystyrene, polypropylene, ABS, polyvinyl chloride, and PETG have muchlower thermal properties. A minimum wall thickness of 0.012 to 0.015inches is required and working distances of from 14 inches to 18 inchesshould be maintained. In the cases of polypropylene and PVC atwall-thicknesses of 0.015 inch, it is also advisable to reduce the powersetting to the evaporant source by twenty-five percent and increase thetime cycle by twenty-five percent to prevent warpage (primarily due to“hot spots” in the vapor flux). Graph B shows relative coating thicknessas a function of vapor source-to-part distance comparing injectionmolded parts to various thermoforms.

[0075] It should be understood that thin-walled thermoforms frompolymeric sheet of thicknesses from 0.006 inches to 0.100 inches arecontemplated by this invention to be metal coated and used for EMIshielding, as are thick-walled theroforms having wall thickness inexcess of 0.100 inches.

[0076] Recycling

[0077] Two options for recycling metallized inserts are available. Thefirst is chemical stripping and the second is simply regrinding andre-extruding the scrap for reuse.

[0078] Vacuum deposited aluminum is easily removed with solutions ofpotassium and sodium hydroxide. These spent solutions containingaluminum can be diluted or neutralized with acid. Solutions with a pHunder twelve which contain no heavy metals can be released to a sanitarysewer system without any treatment. Other deposited metals requirepretreatment depending on their concentrations.

[0079] A better alternative is to simply shred and regrind themetallized thermoforms. This material can then be re-extruded into rollor sheet form (as would also be done with the trimmed metallized scrap).The re-extruded material has thus been filled with metal. This materialis used to form new inserts which would be deposited with metal on theirexterior surfaces again. In effect the material becomes more conductivethe more it is recycled. Recycling would be the recommended manner ofdisposing of inserts metallized with metals and alloys other thanaluminum.

EXAMPLE

[0080] Eight samples were tested for shielding effectiveness using the“modified MIL-STD-285” method. In this procedure, samples to be testedare mounted in a test opening in the wall of shield room. The tests arerun by radiating the test samples with a signal generator and antennainside the room, and measuring the levels outside the shield room with aspectrum analyzer and antenna. A baseline measurement was made throughthe open hole, without any samples in place. The difference in these twomeasurements-before and after the samples are installed in thehole-yields the “shielding effectiveness.”

[0081] Certain errors can be introduced in this method at lowerfrequencies. The “hole” itself provides shielding when the longestdimension is less than one half wavelength, but since values aresubtracted from the “baseline”, the errors give more conservative lowerlevels of shielding than free space measurements. In this case, theseerrors only affected the 30 and 50 MHz measurements, making thosemeasurements more conservative by 15 dB. The remaining measurements,from 60 MHz to 3 GHz, are not affected. (NOTE: This is not consideredsignificant, but is included for clarity.)

[0082] Tests were performed in three ranges, as follows:

[0083] Range I—30 MHz-200 MHz. Test equipment consisted of biconicalantennas, signal generator/amplifier, and spectrum analyzer. Asdiscussed earlier, some reduction is caused by the “hole” at frequenciesbelow 60 MHz.

[0084] Range II—200 MHz-1 GHz. Test equipment consisted of log periodantennas, signal generator, and spectrum analyzer. No reductions due to“hole” effects.

[0085] Range III—1 GHz-3 GHz. Test equipment consisted of microwave hornantennas, microwave signal generator, and spectrum analyzer. Noreductions due to “hole” effects.

[0086] Electric field measurements were made on all samples at selectedfrequencies from 30 MHz to 3 GHz. This data is summarized in Table 2,and FIGS. 1-5.

[0087] Surface impedance measurements were made on all samples using aKeithly micro-ohmmeter. Measurements were made approximately one inchapart at nine locations on each sample (top-middle-bottom byleft-center-right). The nominal averages are summarized in Table 1. Aprediction for plane wave shielding is also included, based on thefollowing formula:

[0088] SE=20 log (Zw/4 Zs)

[0089] SE is the shielding effectiveness in dB

[0090] Zw is the wave impedance (assumed as 377 ohms)

[0091] Zs is the surface impedance (ohms/square)

[0092] This formula is a “first approximation”, but the predicted datamay be useful in correlating the test data.

[0093] Finally, magnetic field measurements were made on two samples atselected frequencies from 10 KHz to 20 MHz. Since low frequency magneticfield shielding normally requires thick steel or mu-metals, littleshielding was anticipated.

[0094] TEST RESULTS: The test data is summarized in Table 2, and isshown graphically in FIGS. 1-5.

[0095]FIG. 1 compares the silver acrylic paint with the silver vacuumdeposition coating. The silver vacuum coating performed approximately 10dB better than the silver paint. Both samples #1 and #8 used silverpaint, and show consistent test results. Sample #2 is the silver vacuummaterial.

[0096]FIG. 2 compares the aluminum vacuum coating, one side versus twosides with one coat of material. The double sided sample performsapproximately 10 to 20 dB better. This is believed due to the secondreflection that occurs at the second surface.

[0097]FIG. 3 compares the aluminum coating, one side versus two sideswith two coats of material. These are quite close in performance. It wasnoted that the single side sample had a very low surface impedance (0.03ohms/square) while the double sided sample had higher surface impedances(0.15 ohms/square). Had the surface impedances been the same, a largerdifference would be expected. Nevertheless, one can conclude from thisthat two thin coats perform as well as one thick coat.

[0098]FIG. 4 compares aluminum, silver, and phosphor bronze. All threesamples here were single side, single coat. Not surprisingly, the silverdoes the best.

[0099] For solid shields, copper would normally be expected to performbetter than aluminum. In this case, however, the copper did not appearto be deposited in a uniform manner. Also, the surface impedance wasmuch higher than the silver or aluminum.

[0100] One can conclude from this graph that while the silver does thebest, the aluminum performs quite well. Unless the copper depositionproblems can be overcome, aluminum is preferred.

[0101]FIG. 5 shows the brief magnetic field tests done on two samples.Although very little shielding effectiveness was expected, it appearsthe materials start to provide some magnetic field shielding above 1MHz. This is consistent with other thin non-ferrous materials. (Althoughsometimes “magnetic shielding” is claimed based on this type of data, itis really misleading and should not be done.)

[0102] From the test data, the following conclusions were reached:

[0103] All of the samples performed quite well in the electric fieldtesting. With the exception of the phosphor bronze sample, all othersamples provided 40-60 dB of shielding. This should be more thanadequate for most present commercial computer designs.

[0104] The silver vacuum sample performed better than the silver acrylicpaint, providing over 60 dB of shielding. This would be very useful forhigh performance requirements, such as high speed computers or radiotransmitters/receivers.

[0105] The double sided aluminum samples also performed quite well,providing over 50 dB of shielding. This is believed due to the doublereflections provided by metallizing two surfaces. Metallizing both sideswould be recommended when additional shielding is desired.

[0106] The copper (phosphor bronze) sample did not perform as well asthe aluminum. This may be due to non-uniform deposition on the surface.Given the overall good performance from the aluminum and silver, theremay not be a need for a copper deposited option.

[0107] The lower the surface impedance, the better the shielding. Italso appears that the surface impedance correlates quite well withpredicted and measured shielding effectiveness. While not a finalindicator, surface impedance (ohms/square) is a useful parameter topredict final shielding effectiveness.

[0108] Very little low frequency magnetic field shielding was providedby these samples, although some magnetic field shielding is providedabove 20 MHz. This is not a surprise, since normally this requires thicksteel for shielding.

Having described the invention, I claim:
 1. An EMI shield comprising athin-walled shape formed of thermoformable polymeric material, saidshape having an outer surface and an inner surface and edges, said shapehaving deposited thereon a coating of conductive metal vapor, saidcoating of thickness of approximately 1 to 50 microns.
 2. The EMI shieldof claim 1 wherein said shape is of thickness less than 0.065 inches. 3.The EMI shield of claim 1 wherein said shape has a metal coating on oneor more of the outer surface, the inner surface and the edges thereof.4. The EMI shield of claim 1 wherein said metal coating is vacuumdeposited aluminum vapor.
 5. The EMI shield of claim 1 wherein saidpolymeric material is from the group consisting of polyvinyl chloride,polyethylene terephthalate, acrylonitrile-butenate-styrene, polyimide,liquid crystal polymer, polyetherimide, polysulfone, polycarbonate,polyphenylene sulfide, high-impact polystyrene, glycol-modifiedpolyester, and polypropylene.
 6. The EMI shield of claim 1 wherein saidshape comprises a multiplicity of enclosures joined by webs.
 7. The EMIshield of claim 6 wherein each of said enclosures has a plurality ofsidewalls and an endwall, said enclosures may be folded about said websinto touching engagement therebetween.
 8. The EMI shield of claim 7wherein said enclosures are retained in engagement by conductive means.9. The EMI shield of claim 6 wherein said webs have no metal coatingthereon.
 10. The EMI shield of claim 1 wherein said shape is ofthickness less than 0.065 inches, said shape has a metal coating on oneor more of the outer surface, the inner surface and the edges thereof,said metal coating is aluminum.
 11. The EMI shield of claim 1 whereinsaid shape comprises a pair of enclosures joined by a web, saidenclosures being generally identical, said web comprising a flexiblehinge, a first of said enclosures folded about said hinge tointerconnect with the other of said enclosures, means to retain saidenclosures in electrically conductive interconnection.
 12. A thin-walledpolymeric body for shielding EMI comprising a polygonal shape formedfrom thin polymeric sheet which has been heated and drawn into a mold oronto a die, said shape having inner surfaces and outer surfaces, saidshape having a conductive metal vapor coating on selected surfacesthereof, said coating being of thickness of at least 1 micron.
 13. Thebody of claim 12 wherein said polygonal shape comprises a multiplicityof sidewalls and an endwall, said sidewalls and said endwall are ofthickness less than 0.065 inches.
 14. The body of claim 12 wherein saidpolymeric sheet comprises one of the group consisting of polyvinylchloride, polyethylene terephthalate, acrylonitrile-butenate-styrene,polyimide, liquid crystal polymer, polyetherimide, polysulfone,polycarbonate, polyphenylene sulfide, high-impact polystyrene,glycol-modified polyester, and polypropylene.
 15. The body of claim 12wherein said metal vapor coating is aluminum.
 16. The body of claim 12wherein said shape comprises a pair of substantially similar enclosuresinterconnected by an integral hinge, each of said enclosures havingsidewalls joined by an endwall, said enclosures pivotable about saidhinge into touching engagement of the sidewalls thereof, mechanicalmeans to retain said enclosures in engagement, each of said sidewallsbeing electrically conductive with said other of said sidewalls.
 17. Thebody of claim 16 wherein said engaged enclosures contain an enclosedspace, means for passage of selected electrical signals into the spacewithin said engaged enclosures.
 18. The body of claim 12 wherein saidshape comprises a multiplicity of polygons interconnected by integralwebs.
 19. The body of claim 18 wherein each of said polygons has an openside, each of said webs having no conductive metal coating thereon. 20.The body of claim 14 wherein said metal coating is from 1 to 50 micronsin thickness, said sheet is from 0.006 to 0.065 inches in thickness.